The present invention relates to pre-expanded particles of a propylene resin suitable for use in expasion molding in a mold, a process for preparing the pre-expanded particles, and a process for preparing a formed article by using the pre-expanded particles.
Hitherto, when pre-expanded particles of olefin resins such as a polyethylene are expansion molded in a mold, it is impossible to be retained a blowing agent in the pre-expanded particles for a long term, e.g. about a week unlike pre-expanded particles of a styrene resin. Therefore, an inorganic gas such as air is introduced into the pre-expanded particles under pressure just before molding in a mold to heighten the internal pressure of the pre-expanded particles to a desired value, and then the pre-expanded particles are charged into a mold and are fused together by heating with steam to give a foamed article. When pre-expanded particles of a propylene resin are expansion molded in a mold, since they are low in gas retention property alike pre-expanded particles of the polyethylene, after a pressured gas such as air is introduced and impregnated into the pre-expanded particles just before molding according to a process described in Japanese Unexamined Patent Publication No. 85158/1974, No. 101025/1983 and No. 40334/1986, and the like to give an internal pressure to the pre-expanded particles, the pre-expanded particles are charged into a mold and heated with steam to give a foamed article, in the same manner as in molding of pre-expanded polyethylene resin particles.
As mentioned above, when the pre-expanded particles of the olefin resins are molded in a mold, a procedure for giving an internal pressure to the pre-expanded particles before molding is indispensable, unlike polystyrene pre-expanded particles.
An object of the present invention is to solve the above-mentioned defect.
This and other objects of the present invention will become apparent from the description hereinafter.